The primary purpose for the enlarged supply of clean inbred rodents from the rodent production center contracts is to: maintain the approximately 100000 breeding animals for hybrid production, e.g., B6D2F1, B6C3F1, CD2f1 (these hybrids are used for cancer research throughout the DCT program); and supply cancer research laboratories with the large number of clean inbred animals needed for tumor transplantation. Obviously, it would be impossible to maintain the continuity of the experimental DCT cancer research program without the contribution of the Rodent Production Center contracts.